1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) apparatus used in, for example, resonators and bandpass filters and also to a manufacturing method for this type of SAW apparatus. More specifically, the invention relates to a SAW apparatus having a structure in which an insulating layer is disposed to cover an interdigital (IDT) electrode and also to a manufacturing method for this type of SAW apparatus.
2. Description of the Related Art
DPX or RF filters used in mobile communication systems need to satisfy wide-band and good temperature characteristics. In known SAW apparatuses used in DPX or RF filters, piezoelectric substrates formed of 36°-50°-rotated Y-plate X-propagating LiTaO3 are used. This type of piezoelectric substrate has a temperature coefficient of frequency (TCF) of about −40 to −30 ppm/° C. In order to improve the temperature characteristic, it is known that a SiO2 film having a positive TCF is formed to cover an IDT electrode on a piezoelectric substrate. An example of a manufacturing method for this type of SAW apparatus is shown in FIGS. 109A through 109D.
As shown in FIG. 109A, a resist pattern 52 is formed on a piezoelectric substrate 51 except for an area in which an IDT electrode is to be formed. Then, as shown in FIG. 109B, an electrode film 53, which serves as an IDT electrode, is formed on the entire surface of the piezoelectric substrate 51. Subsequently, by using a resist stripper, the resist pattern 52 and a metallic film attached to the resist pattern 52 are removed, thereby forming an IDT electrode 53A, as shown in FIG. 109C. Then, as shown in FIG. 109D, a SiO2 film 54 is formed to cover the IDT electrode 53A.
For achieving an object other than the improvement of the TCF, another manufacturing method for a SAW apparatus in which an insulating or non-conductive protective film is formed to cover an IDT electrode is disclosed in Japanese Unexamined Patent Application Publication No. 11-186866. FIG. 110 is a schematic sectional view illustrating a SAW apparatus 61 taught in JP 11-186866. In the SAW apparatus 61, an IDT electrode 63 made of Al or an alloy primarily including Al is disposed on an insulating substrate 62. In an area other than an area in which the IDT electrode 63 is disposed, an insulating or non-conductive inter-electrode-finger film 64 is disposed. An insulating or non-conductive protective film 65 is also disposed to cover the IDT electrode 63 and the inter-electrode-finger film 64. In the SAW apparatus 61 disclosed in JP 11-186866, the inter-electrode-finger film 64 and the protective film 65 are made of an insulating material, for example, SiO2, or a non-conductive material, for example, silicone. By forming the inter-electrode-finger film 64, discharging between the electrode fingers caused by a pyroelectric property unique to the piezoelectric substrate 62 can be suppressed.
Japanese Unexamined Patent Application Publication No. 61-136312 teaches the following type of one-port SAW resonator. An electrode made of a metal, such as aluminum or gold, is disposed on a piezoelectric substrate made of quartz or lithium niobate. Then, after a SiO2 film is formed, it is planarized. In this type of resonator, good resonance characteristics can be achieved by planarizing the SiO2 film.
As shown in FIGS. 109A through 109D, in the manufacturing method for SAW apparatuses in which the SiO2 film 54 is formed for improving the TCF, the height of the SiO2 film 54 is different between a portion with the IDT electrode 53A and a portion without the IDT electrode 53A. Because of the differences in the height of the SiO2 film 54, the insertion loss is increased. These differences in height increase as the thickness of the IDT electrode 53A becomes larger. Thus, the thickness of the IDT electrode 53A cannot be increased.
In the SAW apparatus 61 taught in JP 11-186866, after the inter-electrode-finger film 64 is formed between the electrode fingers of the IDT electrode 63, the protective film 65 is formed. Accordingly, the height of the protective film 65 is uniform, unlike the SAW apparatus shown in FIGS. 109A through 109D.
In this configuration, because the inter-electrode-finger film 64 is formed in contact with the IDT electrode 63 which is made of Al or an alloy primarily including Al, a sufficient reflection coefficient is not obtained in the IDT electrode 63, thereby causing the generation of ripples in the resonance characteristics.
Also, in the manufacturing method taught in JP 11-186866, the resist formed on the inter-electrode-finger film 64 must be removed by a resist stripper before forming the protective film 65. In this case, the IDT electrode 63 may be disadvantageously eroded by the resist stripper. This requires the use of erosion-resistant metal for the IDT electrode 63, thereby decreasing flexibility in selecting the type of metal used in the IDT electrode 63.
In the one-port SAW resonator taught in JP 61-136312, quartz or lithium niobate is used for the piezoelectric substrate, and the electrode is made of aluminum or gold. In JP 61-136312, only the embodiment in which the electrode is made of Al and is disposed on a quartz substrate is taught, and no specific reference is made to a SAW apparatus using a substrate made of another type of material or an electrode made of another type of metal.
JP 61-136312 teaches that superior resonance characteristics are achieved by planarizing the SiO2 film. Then, in order to obtain a wide-band filter, the present inventors formed a one-port SAW resonator having a structure similar to the structure taught in JP 61-136312, except that a LiTaO3 substrate having a large electromechanical coupling coefficient was used as the piezoelectric substrate. The present inventors then examined the characteristics of the one-port SAW filter.
More specifically, an Al electrode was formed on the LiTaO3 substrate, and then, a SiO2 film was formed and the surface of the SiO2 film was planarized. However, a considerable deterioration in the characteristics after the formation of the SiO2 film was observed, and the present inventors found that this SAW resonator cannot be put to practical use.
By using a LiTaO3 substrate or a LiNbO3 substrate having a larger electromechanical coupling coefficient than quartz, the fractional bandwidth is increased considerably. However, the present inventors found that, after the formation of an Al electrode on a LiTaO3 substrate and after the formation of a SiO2 film, the reflection coefficient was sharply decreased to about 0.02, as shown in FIGS. 2 and 3, caused by the planarization of the SiO2 film. FIGS. 2 and 3 illustrate the relationship between the reflection coefficient and the thickness H/λ of the IDT electrodes when the IDTs are made of Al, Au, Pt, Cu, and Ag and the SiO2 film were formed on a LiTaO3 substrate having Euler angles (0°, 126°, 0°). The solid lines in FIGS. 2 and 3 represent a relationship between the reflection coefficient and the thickness H/λ of the IDT electrodes when the surface of the SiO2 film was not planarized. The broken lines indicate a relationship between the reflection coefficient and the thickness H/λ of the IDT electrodes when the surface of the SiO2 film was planarized.
FIGS. 2 and 3 show that, when the Al electrode was used, the reflection coefficient is decreased considerably to about 0.02 by the planarization of the surface of the SiO2 film, regardless of the thickness of the IDT electrode. Accordingly, a sufficient stop band cannot be achieved, causing the generation of sharp ripples in the vicinity of the antiresonant frequency.
It is known that the reflection coefficient becomes larger as the thickness of an electrode is increased. As is seen from FIGS. 2 and 3, the reflection coefficient is not increased as the thickness of the Al electrode is increased when the surface of the SiO2 film was planarized.
In contrast, as is seen from FIG. 2, the reflection coefficient is increased as the thickness of the Au or Pt electrode is increased even when the surface of the SiO2 film was planarized.